This invention relates to the injection molding of engineered thermoplastics including fiber reinforced plastics which perform at extremely high temperatures and stresses. The processes and apparatus disclosed herein may also be utilized for injection molding of thermosets.
In previously filed patent applications, including application Ser. No. 10/868,574 entitled Microwave Molding of Polymers, Publication No. US-2004-0222554-A1, and application Ser. No. 10/435,315 entitled Microwave Molding of Polymers, Publication No. US-2003-0224028-A1, I disclosed methods for creating compression molds for use in the compression molding of polymers using microwave energy to heat the polymer to its melting point. The molds and processes disclosed therein are particularly well adapted for molding plastic polymers and composites having a relatively high operating temperature, including such high performance polymers as those sold under the trademarks PEEK®, TORLON®, SEMITRON®, DURATRON®, CELAZOLE®. The use of microwave energy to heat the polymer in the compression molds disclosed therein will result in significant energy savings compared to molding processes using electric, gas or steam heating to heat the polymer to its melting point.
Using the molds formed in the manner disclosed in my prior applications, rapid and uniform heating of thermoplastic and thermoset materials by microwave energy may be achieved due to the volumetric nature of microwave (MW) heating. Polymer material in powder or pellet form is compacted within a mold cavity of the mold assembly which is placed into the resonance cavity of a multimode microwave oven and exposed to microwave radiation. Upon reaching the desired temperature at which the polymer material melts or softened, the mold halves are squeezed together by a hydraulic press to mold the molten polymer into desired shape. The mold is then allowed to cool until the molded polymer solidifies and retains its molded shape. The mold is designed to provide relatively uniform heating of polymer material due to approximately equal heating rates of all of the mold members and polymer or work material resulting in relatively uniform heating of the polymer.
It is believed that the compression molding techniques using microwave energy described in my prior published patent applications provide higher quality finished products, shorter processing times by a factor of approximately 10 or more, and reduced consumption of energy by the same factor. Nevertheless, in spite of significant advantages, microwave compression molding has drawbacks. For example, parts formed from compressed pellets or powders usually have lower mechanical properties than injection molded parts. Also grinding pellets to a fine powder results in increased material and handling costs. Although the use of pellets avoids the additional cost associated with grinding the pellets into fine powder, compression molding of pellets can result in finished product showing visible borders at the interfaces between molten granules. This may be explained by the rheological properties of molten pellets and by the absence of mixing and dispersing mechanisms to obtain homogeneity of the melt. In addition, if the microwave compression molding process is not performed with adequate compression or compaction of the pellets, the resulting product can also have undesirable voids or pores.
Injection molding of plastics from pellets has been widely and effectively used in the plastic industry for decades. In a conventional injection molding machine 1, shown in FIG. 1, a screw 2 rotates in heated barrel 3 in order to melt and inject the plastic material into the mold cavity 4 of mold 5. Pellets of polymer material supplied from hopper 6 into barrel 3 are plasticized by the heat transfer from external heaters 7 through barrel 3 and the heat created by the shear forces generated from rotation of the screw 2 against the pellets in the barrel 3. The molten polymer is forced toward the front of the barrel 3 by the action of the screw 2 such that the molten polymer is forced out of the barrel 3 and injected into the mold cavity 4. Another general type of plasticizing unit is a plunger type plasticizing unit 10 shown in FIG. 2. The plunger type plasticizing unit 10 uses external heaters 11 mounted on the barrel 12 to heat and melt the polymer pellets which are fed into the barrel through hopper 13. Instead of a screw, the plunger type plasticizing unit 10 uses a plunger 14 to force the palletized polymer toward the front of the barrel 12. A torpedo shaped baffle or diverter 15 is positioned in the barrel 12 near the front end thereof which forces the pelletized polymer feed through a relatively thin section to ensure melting of the polymer prior to injection out of a nozzle 16 at the front of the barrel 12 and into mold cavity 17 of mold 18.
At the present time there exist numerous variations of screw and plunger type plasticizing units for injection molding. These injection molding machines utilize conventional heat transfer from the external heaters through the barrel to melt the plastic material in the barrel. Because of their molecular structure, plastics have low thermal conductivities; thus it is difficult to rapidly transmit heat through the polymers by conduction. Known plasticizing units for injection molds therefore incorporate long screws and barrels to increase the internal heating surface area and travel time to allow the polymer to melt prior to injection from the plasticizing unit into the mold.
Due to the low thermal conductivity of plastics, the ratio of the volume of the material injected out of the plasticizing unit to the heating surface area of the plasticizing unit per cycle is relatively small and therefore inefficient. It would be desirable to find a means for significantly increasing the efficiency of heating the pelletized plastic without increasing the size of the plasticizing unit. However, increasing the shot capacity by accumulation of molten plastic in the rear zone of plasticizing unit may cause plastic degradation due to prolonged heating and is not desirable.
Although the use of microwave heating of plastics in a plasticizing unit of an injection molding machine might seem attractive, only a few types of thermoplastics may be effectively heated from room temperature by microwave energy. Conversely, conventional heating techniques employed in current injection molding processes, including electric and steam heating, are capable of processing almost all types of polymers.
The dielectric properties of polymers and their coupling with microwaves are directly related to their molecular structure. Polymers are generally nonpolar, having very regular structures and, therefore, low molecular polarity, so that they exhibit practically no dielectric loss. The negligible dielectric loss of most plastics is the reason why plastic containers do not heat up due to exposure to microwave energy when used to contain items heated in a microwave oven. In contrast to plastics, water molecules have a high polarity which makes water an ideal material for microwave heating and which permits drying of polymers with microwave energy prior to molding. Any water absorbed by the polymers will be heated by the microwave energy and evaporated in a short period of time. After the water has dried out, the polymer will exhibit a true value for its dissipation factor, tanδ, which is the measure of polymer's ability to absorb the microwave radiation. As discussed above, for many polymers the dielectric losses are not sufficient to permit raising of the polymer's temperature from room or ambient temperatures to its melting or softening temperature through microwave heating.
Most of the modern high performance engineering plastics, discussed above, have low or moderate values for their dissipation factors, tanδ. Therefore, microwave processing of such polymers is generally considered impractical, especially for polymers containing glass fillers.
A variety of conveyer belt or drum type systems are available for microwave drying of polymers which may be used to feed an injection molding machine with dry polymer pellets. However, none of these known prior art microwave drying systems are adapted for use in melting or plasticizing the plastic material as part of an injection molding system.
There remains a need in injection molding systems which allow for the rapid and uniform heating of high performance engineered plastics with relatively high operating temperatures.